1. Field of the Invention
This invention pertains to that part of borehole drilling apparatus known as stabilizers and more specifically to a type of stabilizer which can be characterized as a removable, wedge-lock type stabilizer.
2. Description of the Prior Art
Stabilizers, sometimes referred to as drill collar stabilizers or as drill stem stabilizers, have been employed in earth boring operations for the petroleum industry to centralize the drill stem in the borehole, usually especially in the drill collar section at a distance of from 100 feet to 300 feet above the drill bit. The purposes of a stabilizer are to (1) help control hole angle direction, (2) prevent the bit from drifting laterally, which would result in undesirable dog-legs and ledges, and (3) improve bit performance by forcing the bit to centrally rotate about its axis so as to provide substantially equal force loading on all three drill bit cones. In addition, stabilizers also may be used to provide a reaming function for undersized or irregularly shaped boreholes provided the formation is not too hard. Rolling cutter reamers are employed to provide these functions for formations too hard to be thus treated by a stabilizer.
Stabilizers may be further categorized as rotating stabilizers and as non-rotating stabilizers. Non-rotating stabilizers do not rotate as the drill string is turned, its wall-contacting members merely moving longitudinally along the wall as the drill string is lowered and raised. On the other hand, a rotating stabilizer includes wall-contacting members that rotationally track along the wall of the borehole as the drill string is turned. In addition, rotating stabilizers can be further divided into fixed-contact and rolling contact types, which latter type would be functionally equivalent to a roller-reamer.
The contacting members of a fixed-contact type of stabilizer, which is the type of stabilizer described herein, are subjected to the various forces attendant to the entire drill string, the severest of which is often the longitudinal thrust force and the most constant and aggravating of which are the fretting forces. It should be noted that forces applied to the drill string are a result of the drill string manipulations, the conditions of the bore, and the fluid conditions internal and external to the drill string.
One fixed-contact type stabilizer is shown in U.S. Pat. No. 3,454,308, Kennedy, in which two wear bars are positioned end-to-end on either side of a locking bar within an accommodated slot, the wear bars being dovetailed to fit under tapered end surfaces of the slot and a locking bar. The locking bar is secured by a cap screw. Should the cap screw not be adequately tightened or should the cap screw vibrate loose, the wear bars are in danger of falling out during use of the tool.
Another fixed-contact type stabilizer is shown in U.S. Pat. No. 3,818,999, Garrett, in which the wear elements are accommodated in a V-section groove, the wear elements being held in place by cap screws. Only the cap screws provide surfaces for resisting the thrust forces and a broken or loosened cap screw will cause the wear element ordinarily held thereby to be dislodged.
A third type of fixed-contact stabilizer is shown in U.S. Pat. No. 4,106,823, Bassinger, in which tapered pairs of wear pads are wedged side by side in an accommodating slot, the pads being dovetailed along their sides and held thereby by tapered slot side surfaces to secure against lateral dislodging. Such pads are set in place by striking the ends with a mallet or hammer, with variable results depending on how well the tapered surfaces fit together and on the human element. High thrust loading and fretting can loosen and dislodge such pads during use either because the pads are not tightly sealed or because the uneven surfaces do not permit uniform tightening along their entire lengths.
Therefore, it is a feature of the present invention to provide an improved stabilizer in which the wear elements are tapered on the underneath side and secured in place by one or more wedging pieces.
It is another feature of the present invention to provide an improved stabilizer in which there are a plurality of wear elements in a single pocket, each one being separately secured therein by a tapered underneath side and by one or more mating wedge pieces.
It is still another feature of the present invention to provide an improved stabilizer in which there are at least two wear elements in a single pocket that are horizontally aligned, the opposing underneath surfaces of which taper toward one another, thereby permitting one or more wedge pieces to be used therebetween for securing the wear elements in place.